The present invention relates to a method and apparatus for mixing a second medium with a first medium. The method and apparatus of the invention are especially suitable for mixing different chemicals, both fluid and gaseous ones, or steam with a so-called first medium, which is composed of both solid and fluid matter, like for example, cellulose fibre suspensions of the wood-processing industry or mixtures of, e.g., different beet chips (such as potato and sugar beet) and water.
Prior art mixers used for this purpose are disclosed, e.g., in U.S. Pat. Nos. 5,279,709 and 5,575,559 and in patent applications EP-A-92921912, EP-A-9100973, WO-A-96/32186, and WO-A-96/33007. It is a characteristic feature of all mixers of the art that they employ a rotatable rotor in order to provide a sufficient mixing efficiency. The rotatable rotor specifically refers to a member which is connected to the drive through a shaft and most usually receives its power from the electricity supply of the mill. Furthermore, the mixer construction is usually such that a certain pressure loss occurs in the mixer. In practice, it means that the power compensation corresponding to the pressure loss caused by the mixer has been taken into account when selecting a pump which operates at some stage of the process and precedes the mixer. So, in practice, power is lost in the pump for compensating the pressure loss of the mixer as well as in the mixer itself for rotating its rotor.
The method and apparatus in accordance with the invention eliminate one of the power-losing factors mentioned above. The rotor of the mixer is arranged to rotate freely in the flow, whereby the mixer naturally causes a certain pressure loss. However, a thorough research work has given such results that the pressure loss has not increased, at least not essentially, when compared with a motor-driven rotor. Furthermore, in spite of considerable power savings, no change in the mixing result can be found, at least not for the worse.
So-called static mixers are known in the art, but they are mostly of the type disclosed in, e.g., U.S. Pat. Nos. 4,030,969, 5,492,409, and 5,556,200, in which a throttling effect of some degree is arranged in the flow channel, whereby the flow rate increases and the pressure is reduced. The chemical or equivalent to be mixed is then conveyed to this lower-pressure zone, and the turbulence effect, also developed by throttling, then mixes the chemical or equivalent with the actual flow material. Another alternative is disclosed in, for example, U.S. Pat. Nos. 4,936,689 and 5,564,827, where the flow channel is provided with obstacles to flow so as to create turbulence. It is a characteristic feature of both types of mixers that the turbulence created is of local nature only and short in duration.
By the method and apparatus in accordance with the present invention, the kinetic energy originating from the pressure losses caused by the throttling device may be forwarded to the mixing zone in a more controlled manner and to a wider area, which will substantially grow the efficient mixing volume and substantially lengthen the mixing time.
It is a characteristic feature of the method in accordance with the invention of mixing a second medium with a first medium, in which method the first medium is introduced into a casing of a mixing apparatus, where it is mixed and discharged therefrom, that the rotor of the mixer disposed in the casing is rotated by a medium flow entering the casing.
It is a characteristic feature of the apparatus according to the invention, for mixing a second medium with a first medium, which apparatus comprises a mixer casing with an inlet and an outlet, both of these having a flange, and with a rotor, that the rotor is freely rotatable.
Other aspects characteristic of the method and apparatus of the invention will become apparent from the attached claims.
The method and apparatus in accordance with the invention are described more in detail below, by way of example, with reference to the enclosed drawings: in which